Seals are used between inner and outer wellhead tubular members to contain internal well pressure. The inner wellhead member may be a casing hanger located in a wellhead housing and that supports a string of casing extending into the well. A seal or packoff seals between the casing hanger and the wellhead housing. Alternatively, the inner wellhead member could be a tubing hanger that supports a string of tubing extending into the well for the flow of production fluid. The tubing hanger lands in an outer wellhead member, which may be a wellhead housing, a Christmas tree, or a tubing head. A packoff or seal seals between the tubing hanger and the outer wellhead member.
A variety of seals located between the inner and outer wellhead members have been employed in the prior art. Prior art seals include elastomeric and partially metal and elastomeric rings. Prior art seal rings made entirely of metal for forming metal-to-metal seals (“MS”) are also employed. The seals may be set by a hydraulically activated running tool, or they may be set in response to the weight of the string of casing or tubing. One type of prior art metal-to-metal seal has seal body with inner and outer walls separated by a cylindrical slot, forming a “U” shape. An energizing ring is pushed into the slot in the seal to deform the inner and outer walls apart into sealing engagement with the inner and outer wellhead members, which may have wickers formed thereon. The energizing ring is typically a solid wedge-shaped member. The deformation of the seal's inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent. However, the portion of the inner and outer seal walls may not provide the best seal possible because the metal comprising the seal is relatively hard. A dilemma however exists because the seal must also be able to handle the mechanical loads it is subjected to.
A need exists for a technique that addresses the seal issues described above. In particular, a need exists for a technique to improve the sealing capability of seals without compromising the load capacity of the seal. The following technique may solve these problems.